Consider re-ordering arguments for precipitable_water

[dopplershift]

So the call for precipitable water is:

precipitable_water(dewpt, pressure, bottom=None, top=None)

We should probably considering changing this to:

precipitable_water(pressure, dewpt, *, bottom=None, top=None)

This is re-ordering the first two arguments for consistency. The other is to change (now that we're on Python 3) to make bottom and top kwarg-only.

[dopplershift]

Marking 0.12 to put in a shim in future to allow using it in 0.12.

[dopplershift]

We may also want to consider doing this for storm_relative_helicity.